1. Field of the Invention
This invention relates to a method and apparatus for calibrating a first co-ordinate frame of an indexing means such as a robot and end effector in a second frame of reference of a sensing means such as a sensor when the sensing means is substantially rigidly attached to the indexing means, particularly, but not exclusively suitable for use in robotic drilling operations.
2. Discussion of Prior Art
In any system involving automated movement of parts where the movement is controlled by an external sensor system, the relationship between the body performing the movement and the sensor system must be known in order to effect accurate robot motion. Typical sensor systems used with robots either require contact with or are remote from the robot and parts to be worked. The contact systems involve intensive alignment and are sensitive to environmental variations. The majority of six degree of freedom (6 DOF) non-contact configurations currently used have digital or film based cameras located remote from the robot, and typically a polar measurement device, such as a laser tracker, is used for calibration purposes instead of the 6 DOF system, introducing ancillary equipment and thus increasing costs through hardware and set-up time.
There is therefore a need for a generally improved method and apparatus for calibrating a first co-ordinate frame of an indexing means in a second frame of reference of a sensing means, where the sensing means is substantially rigidly attached to the indexing means, and thus utilises parts that will be used for on-line measurements.
According to a first aspect of the present invention there is provided a method for calibrating a first co-ordinate frame of an indexing means in a second frame of reference of a sensing means when the sensing means is substantially rigidly attached to the indexing means, including the steps of measuring first positions of a plurality of first light sources relative to the sensing means, each of which first light sources is remote from the sensing means and is either an active light source or an illuminable reflecting point, with the first positions corresponding to a first absolute position of the indexing means, moving the indexing means, measuring second positions of the plurality of first light sources relative to the sensing means, which second positions correspond to the desired second absolute position of the indexing means, repeating the measurement of and movement from the first to second positions so as to provide at least two sets of measurements of first and second positions corresponding to at least two movements, each of which at least two movements is a known, unique transform, combining the at least two sets of measurements of first and second positions of the plurality of first light sources relative to the sensing means by a transformation means such as a transformer so as to calibrate the first co-ordinate frame in the second frame of reference.
Preferably the measurements of first and/or second positions of the plurality of first light sources relative to the sensing means includes the steps of imaging on at least two imaging devices light reflected or projected from the plurality of first light sources, transmitting signals indicative of the distribution of reflected or projected light from each of the at least two imaging devices to a processor and combining the same so as to define the first and/or second positions of the plurality of first light sources relative to the sensing means.
Conveniently the known movements corresponding to the at least two sets of first and second positions are two translations.
Advantageously the known movements corresponding to the at least two sets of first and second positions are one translation and one rotation.
Preferably the known movements corresponding to at least two sets of first and second positions are two rotations about different axes, with or without at least one translation and/or further rotation.
Conveniently the combination of the at least two sets of first and second position measurements includes, for each of the at least two sets of first and second position measurements, combining each of the first and second measured positions with a matrix multiplication of the corresponding known movement transform and an unknown transform, which unknown transform describes the first co-ordinate frame in the second frame of reference to give a set of at least two expressions, summing each of the set of at least two expressions, and optimizing any errors resulting therefrom by standard optimization techniques, in order to determine the unknown transform, which unknown transform calibrates the first co-ordinate frame in the second frame of reference.
According to another aspect of the present invention there is provided apparatus for calibrating a first co-ordinate frame of an indexing means in a second frame of reference of a sensing means when the sensing means is substantially rigidly attached to the indexing means, including sensing means, indexing means for moving the sensing means when the sensing means is substantially rigidly attached thereto, a plurality of first light sources, which first light sources include active light sources or illuminable reflecting points and are locatable at a fixed position in space, a processor in operative association with the indexing means, transformation means in operative association with the processor, transmitting means for transmitting signals indicative of a first and a second position of each of the plurality of first light sources relative to the sensing means from the sensing means to the processor, which first and second positions of each of the plurality of first light sources relative to the sensing means correspond to an absolute first position and a desired, absolute second position of the indexing means respectively, such that when the indexing means moves from the absolute first position to the desired absolute second position, the first and second positions of each of the plurality of first light sources relative to the sensing means transmitted to the processor are thence combined by the transformation means so as to calibrate the first co-ordinate frame in the second frame of reference.
In a preferred embodiment the indexing means is a robot and an end effector the sensing means is a sensor and the transformation means is a transformer.
Conveniently the sensing means includes at least two imaging devices, each attachable to the end effector and each configurable to image each of the plurality of first light sources.
Advantageously each of the at least two imaging devices is a metrology sensor operable to create digitizable images, and which at least two imaging devices are substantially equi-spaced around the end effector.
Preferably there are at least two second light sources, each associated with a respective imaging device, and wherein the plurality of first light sources is a plurality of reflective targets.
Conveniently there is provided a first part, which first part carries the plurality of light sources and includes a rectangular plate, a shaft of substantially circular cross-section, which shaft is removably attachable to a first face of the rectangular plate and has at least one of the plurality of first light sources positioned thereon, and means for removably attaching the shaft to the rectangular plate.
Advantageously the transmitting means includes coaxial cables and frame grabber ports for transmitting a signal indicative of the first and a second positions of each of the plurality of first light sources relative to the sensing means to the processor.